so how did Bell and Case first meeting go?
Depends on the au we talking about.
Canon- After Case died, him and Bell just kinda got stuck in limbo together. Case opened his eyes, seeing Harrow's and his bodies floating, feeling a tug back to the Rook. To Adler, and that's where he saw Bell, just following Adler sadly. both stuck as ghosts, haunting Adler for the rest of his days. So in short, not very well.
Ghost Bell AU- Case woke up from a nightmare one of the team's first nights at the Rook. And there Bell was, standing in the cornet. Just watching. Scared the shit out of poor Case.
Bell Lives AU- To paraphrase basically what has happened in my fic so far, Case and Bell have only kinda met, mostly just Case bc Bell wasn't quite all there when Case came in, but it was mainly just confusion all around. I'll get to how they officially meet written... eventually....